linux-kernel - Re: [PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT

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Date:   Wed, 10 Jun 2020 11:32:19 +0100
From:   Will Deacon <will@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Naohiro Aota <naohiro.aota@....com>,
        Stephen Boyd <swboyd@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Robin Murphy <robin.murphy@....com>,
        Dave Martin <Dave.Martin@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT_VDSO

On Wed, Jun 10, 2020 at 11:29:17AM +0100, Catalin Marinas wrote:
> On Wed, Jun 10, 2020 at 09:47:55AM +0100, Will Deacon wrote:
> > On Tue, Jun 09, 2020 at 04:55:13PM -0700, Nick Desaulniers wrote:
> > > On Tue, Jun 9, 2020 at 1:35 PM Catalin Marinas <catalin.marinas@....com> wrote:
> > > > On Mon, Jun 08, 2020 at 01:57:08PM -0700, Nick Desaulniers wrote:
> > > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > > index 7f9d38444d6d..fe9e6b231cac 100644
> > > > > --- a/arch/arm64/Kconfig
> > > > > +++ b/arch/arm64/Kconfig
> > > > > @@ -1299,6 +1299,14 @@ config COMPAT_VDSO
> > > > >         You must have a 32-bit build of glibc 2.22 or later for programs
> > > > >         to seamlessly take advantage of this.
> > > > >
> > > > > +config THUMB2_COMPAT_VDSO
> > > > > +     bool "Compile the vDSO in THUMB2 mode"
> > > > > +     depends on COMPAT_VDSO
> > > > > +     default y
> > > > > +     help
> > > > > +       Compile the compat vDSO with -mthumb -fomit-frame-pointer if y, otherwise
> > > > > +       as -marm.
> > > >
> > > > Now that we understood the issue (I think), do we actually need this
> > > > choice? Why not going for -mthumb -fomit-frame-pointer always for the
> > > > compat vdso?
> > > 
> > > "Why should the compat vdso be configurable?" is a fair question.  I
> > > don't have an answer, but maybe some of the folks on thread do?
> > > 
> > > Our problem is more so "if the vdso is built as thumb, we need it also
> > > explicitly built with -fomit-frame-pointer."  Whether it should be
> > > built as thumb, arm, or configurable (and which default to pick in
> > > that case) are still an open questions.  Will asked for it to be
> > > configurable, so I sent a patch making it configurable.
> > 
> > It's configurable for 32-bit arm,
> 
> On 32-bit, the vdso mode is a side-effect of how we build the kernel
> image. I guess we haven't put much thought into whether we want to keep
> the vdso in Thumb-2 or ARM mode.

I think your guess is correct!

> > so I was just following that as it's
> > hardly a maintenance burden to support both. I suppose you could have
> > a toolchain that only supports one or the other, but it does seem a little
> > esoteric if you're building a kernel for an arm64 CPU.
> 
> We could leave the config option in if we ever need to change the compat
> vdso mode. But as not to confuse others with too many options, maybe
> add:
> 
> 	bool "Compile the vDSO in THUMB2 mode" if EXPERT
> 
> Either way:
> 
> Acked-by: Catalin Marinas <catalin.marinas@....com>

Cheers, I'll add the dependency on EXPERT since I agree it's probably not
something most people would want to touch. I'll also change the text to say
"compat vDSO" not just vDSO.

Will